Method and apparatus for mitigating pilot pollution in a wireless network

ABSTRACT

Techniques for mitigating pilot pollution in a wireless network are described. In an aspect, pilot pollution may be mitigated by reducing density and/or transmit power of common pilots whenever possible. A cell may send a common pilot at a first density and a first transmit power level during a first time period and may send the common pilot at a second density and a second transmit power level during a second time period. The second density may be lower than the first density and/or the second transmit power level may be lower than the first transmit power level. Lower density may be achieved by sending the common pilot less frequently, on fewer subcarriers, and/or from fewer antennas. The cell may determine whether to reduce the density and/or transmit power of the common pilot based on network loading, SINRs of terminals, etc. In another aspect, pilot pollution may be mitigated by performing pilot cancellation at a terminal.

The present application is a continuation of, and claims priority to,U.S. application Ser. No. 12/359,992, filed on Jan. 26, 2009 andentitled “METHOD AND APPARATUS FOR MITIGATING PILOT POLLUTION IN AWIRELESS NETWORK,” which in turn claims priority to provisional U.S.Application Ser. No. 61/024,891, entitled “METHOD AND APPARATUS FORMITIGATING PILOT POLLUTION,” filed Jan. 30, 2008. Both of theaforementioned applications are assigned to the assignee hereof andincorporated herein by reference.

BACKGROUND

I. Field

The present disclosure relates generally to communication, and morespecifically to techniques for mitigating adverse impact due to pilotsin a wireless communication network.

II. Background

Wireless communication networks are widely deployed to provide variouscommunication content such as voice, video, packet data, messaging,broadcast, etc. These wireless networks may be multiple-access networkscapable of supporting multiple users by sharing the available networkresources. Examples of such multiple-access networks include CodeDivision Multiple Access (CDMA) networks, Time Division Multiple Access(TDMA) networks, Frequency Division Multiple Access (FDMA) networks,Orthogonal FDMA (OFDMA) networks, and Single-Carrier FDMA (SC-FDMA)networks.

A wireless communication network may include a number of base stationsthat can support communication for a number of terminals. Each basestation may support one or more cells and may periodically transmit oneor more common pilots for each cell. A common pilot is a transmissionthat is known a priori by the terminals and is not associated with atransmission to a specific terminal. The common pilots from the cellsmay be used by the terminals for various purposes such as channelestimation, channel quality measurement, signal strength measurement,time/frequency tracking, etc. Information obtained from the commonpilots (e.g., a channel estimate, a signal strength measurement, etc.)may be used for various tasks such as demodulation and decoding, servingcell selection, etc. It is desirable to send and receive the commonpilots in a manner to achieve good performance.

SUMMARY

Techniques for mitigating pilot pollution in a wireless communicationnetwork are described herein. Pilot pollution refers to degradation inperformance due to interference from common pilots sent by neighborcells. A common pilot may be a cell-specific reference signal used fordemodulation and decoding, a synchronization signal used for cellacquisition, etc.

In an aspect, pilot pollution may be mitigated by reducing densityand/or transmit power of common pilots whenever possible. Density refersto how often a common pilot is sent and may be quantified by time,frequency, space, etc. In one design, a cell may send a common pilot ata first density and a first transmit power level during a first timeperiod and may send the common pilot at a second density and a secondtransmit power level during a second time period. The second density maybe lower than the first density and/or the second transmit power levelmay be lower than the first transmit power level. Lower density may beachieved by sending the common pilot less frequently, on fewersubcarriers, from fewer antennas, or a combination thereof. The cell maydetermine whether to reduce the density and/or transmit power of thecommon pilot based on network loading, offered loading at the cell,measured pilot pollution by terminals, signal-to-noise-and-interferenceratios (SINRs) of terminals, pilot requirements of terminals, and/orother metrics.

In another aspect, pilot pollution may be mitigated by performing pilotcancellation at a terminal. In one design, the terminal may determinewhether to perform pilot cancellation for a neighbor cell. For example,the terminal may decide to perform pilot cancellation if the neighborcell is one of M strongest neighbor cells, if the received signalstrength for the neighbor cell exceeds a threshold, if the neighbor cellis identified by a serving cell, if network loading is light, if arequired SINR of the terminal is high, etc. The terminal may cancel acommon pilot from the neighbor cell if a determination is made toperform pilot cancellation. For pilot cancellation, the terminal mayfirst obtain a channel estimate for the neighbor cell based on areceived signal at the terminal. The terminal may then estimateinterference due to the common pilot from the neighbor cell based on alocally generated common pilot and the channel estimate. The terminalmay subtract the estimated interference from the received signal toobtain an interference-canceled signal. The terminal may perform pilotcancellation for other common pilots from the same neighbor cell and/orfrom other neighbor cells.

Various aspects and features of the disclosure are described in furtherdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication network.

FIG. 2 shows example transmission of common pilots for one cell.

FIGS. 3A and 3B show transmission of a cell-specific reference signalfrom two antennas and four antennas, respectively.

FIG. 4 shows a process for sending common pilots.

FIG. 5 shows an apparatus for sending common pilots.

FIG. 6 shows a process for receiving common pilots.

FIG. 7 shows an apparatus for receiving common pilots.

FIG. 8 shows a process for performing pilot cancellation.

FIG. 9 shows an apparatus for performing pilot cancellation.

FIG. 10 shows a block diagram of a base station and a terminal.

DETAILED DESCRIPTION

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. cdma2000 coversIS-2000, IS-95 and IS-856 standards. A TDMA network may implement aradio technology such as Global System for Mobile Communications (GSM).An OFDMA network may implement a radio technology such as Evolved UTRA(E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, Flash-OFDM®, etc. UTRA and E-UTRA are part ofUniversal Mobile Telecommunication System (UMTS). 3GPP Long TermEvolution (LTE) is an upcoming release of UMTS that uses E-UTRA, whichemploys OFDMA on the downlink and SC-FDMA on the uplink. UTRA, E-UTRA,UMTS, LTE and GSM are described in documents from an organization named“3rd Generation Partnership Project” (3GPP). cdma2000 and UMB aredescribed in documents from an organization named “3rd GenerationPartnership Project 2” (3GPP2). The techniques described herein may beused for the wireless networks and radio technologies mentioned above aswell as other wireless networks and radio technologies. For clarity,certain aspects of the techniques are described below for LTE.

FIG. 1 shows a wireless communication network 100, which may include anumber of base stations 110 and other network entities. A base stationmay be a station that communicates with the terminals and may also bereferred to as an access point, a Node B, an evolved Node B (eNB), etc.A base station may provide communication coverage for a particulargeographic area. The overall coverage area of a base station may bepartitioned into smaller areas, and each smaller area may be served by arespective base station subsystem. The term “cell” can refer to acoverage area of a base station and/or a base station subsystem servingthis coverage area.

A base station may provide communication coverage for a macro cell, apico cell, a femto cell, or some other type of cell. A macro cell maycover a relatively large geographic area (e.g., several kilometers inradius) and may support communication for terminals with servicesubscription in the wireless network. A pico cell may cover a relativelysmall geographic area and may support communication for all terminalswith service subscription. A femto cell may cover a relatively smallgeographic area (e.g., a home) and may support communication forterminals having association with the femto cell (e.g., terminalsbelonging to residents of the home).

Terminals 120 may be dispersed throughout wireless network 100, and eachterminal may be stationary or mobile. A terminal may also be referred toas an access terminal (AT), a mobile station (MS), a user equipment(UE), a subscriber unit, a station, etc. A terminal may be a cellularphone, a personal digital assistant (PDA), a wireless modem, a wirelesscommunication device, a handheld device, a laptop computer, a cordlessphone, a wireless local loop (WLL) station, etc. A terminal maycommunicate with a base station via the downlink and uplink. Thedownlink (or forward link) refers to the communication link from thebase station to the terminal, and the uplink (or reverse link) refers tothe communication link from the terminal to the base station.

In FIG. 1, a solid line with a single arrow indicates a desiredtransmission from a serving cell to a terminal. A dashed line with asingle arrow indicates an interfering transmission from a neighbor cellto a terminal. A serving cell is a cell designated to serve a terminalon the downlink and/or uplink. Uplink transmissions are not shown inFIG. 1 for simplicity.

In wireless network 100, each base station 110 may periodically transmitone or more common pilots for each cell in that base station. A commonpilot may also be referred to as a reference signal, a common referencesignal, etc. Different types of common pilot may be used for differentpurposes and may be referred to by different names. For example, acommon pilot used for channel estimation, channel quality measurement,signal strength measurement, and time/frequency tracking may be referredto as a cell-specific reference signal, a sounding signal, a trainingsignal, etc. A common pilot used for cell acquisition may be referred toas a synchronization signal.

FIG. 2 shows example transmission of common pilots for one cell in LTE.The transmission timeline for the downlink may be partitioned into unitsof radio frames. Each radio frame may have a predetermined duration(e.g., 10 milliseconds (ms)) and may be partitioned into 10 subframeswith indices of 0 through 9. Each subframe may include two slots. Eachradio frame may thus include 20 slots with indices of 0 through 19. Eachslot may cover a fixed or configurable number of symbol periods, e.g.,six symbol periods for an extended cyclic prefix (not shown in FIG. 2)or seven symbol periods for a normal cyclic prefix (as shown in FIG. 2).

For the normal cyclic prefix, each slot includes seven symbol periodswith indices of 0 through 6. A cell-specific reference signal (denotedas REF) may be sent in symbol periods 0 and 4 of each slot in each radioframe. A primary synchronization signal (denoted as PSC) and a secondarysynchronization signal (denoted as SSC) may be sent in symbol periods 6and 5, respectively, of each of slots 0 and 10 in each radio frame. Thecell-specific reference signal and the synchronization signals for LTEmay be generated and sent as described 3GPP TS 36.211, entitled “EvolvedUniversal Terrestrial Radio Access (E-UTRA); Physical Channels andModulation,” which is publicly available.

In general, any number of common pilots may be sent for each cell, andeach common pilot may be sent at any periodicity. The common pilots maycomprise the cell-specific reference signal and the primary andsecondary synchronization signals in LTE. The common pilots may alsocomprise a forward link common pilot, time division multiplexed (TDM)pilots, and a beacon pilot in UMB. The common pilots may also compriseother signals in other wireless networks and other radio technologies.

Wireless network 100 may utilize orthogonal frequency divisionmultiplexing (OFDM) and/or single-carrier frequency divisionmultiplexing (SC-FDM). OFDM and SC-FDM partition the system bandwidthinto multiple (K) orthogonal subcarriers, which are also commonlyreferred to as tones, bins, etc. Each subcarrier may be modulated withdata. In general, modulation symbols are sent in the frequency domainwith OFDM and in the time domain with SC-FDM. The spacing betweenadjacent subcarriers may be fixed, and the total number of subcarriers(K) may be dependent on the system bandwidth. For example, K may beequal to 128, 256, 512, 1024 or 2048 for system bandwidth of 1.25, 2.5,5, 10 or 20 MHz, respectively. The system bandwidth may also bepartitioned into subbands, and each subband may cover a predeterminednumber of subcarriers, e.g., 72 subcarriers in LTE.

FIG. 3A shows example transmission of the cell-specific reference signalfrom two antennas for one cell with the normal cyclic prefix.Time-frequency resources available for transmission may be partitionedinto resource blocks, with each resource block covering 12 subcarriersin one slot. Each resource block may include 84 resource elements forthe normal cyclic prefix, and each resource element may correspond toone subcarrier in one symbol period. For simplicity, only two resourceblocks in two slots of one subframe are shown in FIG. 3A. A firstsubcarrier set may include the first and seventh subcarriers in eachresource block (counting from the bottom in FIG. 3A), and a secondsubcarrier set may include the fourth and tenth subcarriers in eachresource block.

For antenna 0, the cell-specific reference signal may be sent on thefirst subcarrier set in symbol period 0 of each slot and on the secondsubcarrier set in symbol period 4 of each slot. For antenna 1, thecell-specific reference signal may be sent on the second subcarrier setin symbol period 0 of each slot and on the first subcarrier set insymbol period 4 of each slot. For each antenna, no transmission may besent on resource elements used to send the cell-specific referencesignal from the other antenna.

FIG. 3B shows example transmission of the cell-specific reference signalfrom four antennas for one cell with the normal cyclic prefix. Thecell-specific reference signal may be sent from antennas 0 and 1 asdescribed above for FIG. 3A. For antenna 2, the cell-specific referencesignal may be sent on the first subcarrier set in symbol period 1 ofeach even-numbered slot and on the second subcarrier set in symbolperiod 1 of each odd-numbered slot. For antenna 3, the cell-specificreference signal may be sent on the second subcarrier set in symbolperiod 1 of each even-numbered slot and on the first subcarrier set insymbol period 1 of each odd-numbered slot. For each antenna, notransmission may be sent on resource elements used to send thecell-specific reference signal from other antennas.

As shown in FIGS. 2, 3A and 3B, different types of common pilot may besent for each cell. The common pilots may have relatively low overhead(in terms of bandwidth and power) and may be used by all terminals forvarious purposes. However, the common pilots may cause pilot pollution,which may occur due to more common pilots being sent than necessary fora given operating scenario. Pilot pollution may degrade performance indifferent manners depending on how the common pilots are sent. Forexample, pilot pollution may result in the following:

-   -   Poor channel estimation performance and inaccurate interference        estimation due to common pilots from neighbor cells colliding        with common pilots from a serving cell, and    -   Lower received signal quality for data transmission due to        common pilots from neighbor cells colliding with data from the        serving cell.

Received signal quality may be quantified by SINR, signal-to-noise ratio(SNR), carrier-to-interference ratio (C/I), etc. For clarity, SINR isused to denote received signal quality in much of the descriptionherein.

In an aspect, pilot pollution may be mitigated by reducing the densityand/or transmit power of common pilots whenever possible. Sending thecommon pilots with higher density and/or higher transmit power mayimprove performance of channel estimation, demodulation and decoding,channel quality measurement, signal strength measurement, rateprediction, channel quality indicator (CQI) reporting, etc. However,sending the common pilots with higher density and/or higher transmitpower may also increase the likelihood and/or severity of pilotpollution. The density and/or transmit power of the common pilots may bereduced whenever possible based on a tradeoff between performance andpilot pollution.

In one design, the density and/or transmit power of the common pilotsmay be selectively reduced based on network loading. For moderate andheavy network loading, the common pilots may be sent in the normalmanner, e.g., at a nominal density and a nominal transmit power level.For light/low network loading, the common pilots may be sent in areduced manner, e.g., at a lower density and/or a lower transmit powerlevel. Network loading may be quantified by the number of terminalsbeing served, the total amount of data for all terminals being served,the total amount of resources (e.g., bandwidth) used for the terminalsbeing served, the percentage of resources used for the terminals beingserved, etc. One or more thresholds may be used to determine whether thenetwork loading is light, moderate, heavy, etc. Transmission of thecommon pilots may then be controlled based on the detected networkloading. For example, a single threshold may be used to determinewhether or not the network loading is light, e.g., due to only smallsignaling messages and/or VoIP frames being sent. The common pilots maybe (i) sent in the normal manner if the network loading is not light or(ii) sent with lower density and/or lower transmit power if the networkloading is light.

In another design, the density and/or transmit power of the commonpilots may be selectively reduced based on pilot pollution observed byterminals. A terminal may observe high SINR due to light networkloading, low duty cycle of data and possibly control transmissions inneighbor cells, etc. For terminals observing or requiring high SINR,even a small amount of interference due to common pilots from neighborcells may cause a substantial loss in SINR. This may be especially truein an interference-limited deployment in which transmissions fromneighbor cells dominate over thermal noise level observed by theterminals. The degradation in SINR due to pilot pollution may become aprimary limitation of user experience in a partially loaded network.SINR degradation may be dependent on apilot-interference-to-total-interference-and-noise ratio(I_(pilot)/I_(total)). The total interference and noise (I_(total)) mayinclude thermal noise, interference from data transmission, andinterference from common pilots (I_(pilot)). I_(pilot)/I_(total) may beused as a metric for pilot pollution observed by a terminal. High SINRmay be a special case when the total interference and noise is low. Inany case, the common pilots may be reduced when the terminals beingserved observe high pilot pollution (e.g., high I_(pilot)/I_(total)). Athreshold (e.g., an I_(pilot)/I_(total) threshold) may be used todetermine whether or not a terminal observes high pilot pollution. Inone design, the neighbor cells may be requested to reduce the densityand/or transmit power of their common pilots when the terminals beingserved observe high pilot pollution. In another design, the serving cellmay reduce the density and/or transmit power of its common pilots whenthe terminals being served observe high pilot pollution.

The density and/or transmit power of the common pilots may beselectively reduced based on other criteria besides network loading andobserved pilot pollution. For example, the density and/or transmit powerof the common pilots may be selectively reduced based on offered loadingat a serving cell, SINRs of terminals, pilot requirements of terminals,user interference ratio at neighbor cells, etc. The offered loading atthe serving cell may be given by a maximum data rate supported by thecell, the amount of radio resources available in the cell, etc.

The density and/or transmit power of the common pilots may be reduced invarious manners. In one design, the transmit power of the common pilotsmay be reduced by sending the common pilots at a lower transmit powerlevel. For example, the cell-specific reference signal may be sent at anominal transmit power level under normal operating scenarios and at alower transmit power level for light network loading and/or high SINRscenarios. The transmit power level may be given by a power spectraldensity (PSD), a transmit power relative to one milli-Watt (dBm), etc.For example, the PSD of the cell-specific reference signal may bereduced when no downlink traffic is served and/or the served terminalsobverse good SINR conditions.

In another design, the density of the common pilots may be reduced bysending the common pilots less frequently in time. For example, thecell-specific reference signal may be sent in every N-th slot instead ofevery slot, where N may be any value greater than one. The cell-specificreference signal may also be sent in fewer symbol periods (e.g., onesymbol period) in each slot in which it is sent. The cell-specificreference signal may also be sent with data and/or control transmissionsand may be muted or disabled otherwise.

In yet another design, the density of the common pilots may be reducedby sending the common pilots on fewer subcarriers across frequency. Forexample, the cell-specific reference signal may be sent on every L-thsubcarriers, where L may be any value greater than 6. The cell-specificreference signal may also be sent on a portion of the system bandwidthinstead of across the entire system bandwidth.

In yet another design, the density of the common pilots may be reducedby sending the common pilots from fewer antennas and/or reducing thedensity and/or transmit power of the common pilots on each antenna. Thecommon pilots may be sent from all antennas under normal operatingscenarios and from fewer antennas for light network loading and/or highSINR scenarios. For example, the common pilots may be reduced from fourantennas down to two antennas or one antenna, or reduced from twoantennas down to one antenna. The common pilots on each antenna may alsobe reduced by sending the common pilots less frequently, on fewersubcarriers, and/or at a lower transmit power level. The density and/ortransmit power of the common pilots may be reduced by the same amountfor all antennas or by different amounts for different antennas. Forexample, the common pilots on antenna 1 may be reduced more than thecommon pilots on antenna 0.

In one design, the common pilots may be divided into two portions, e.g.,a configurable portion and a non-configurable portion. The configurableportion may be reduced based on any of the criteria described above,e.g., network loading, observed pilot pollution, etc. Thenon-configurable portion may be maintained fixed. For example, thecommon pilots may be divided into a non-loading based portion and aloading-based portion. The loading-based portion may be reduced based onlow or no traffic, the terminals being served not requiring high pilotdensity, etc.

In one design, the common pilots may be sent continually on one or moreantennas, which may be a subset of the available antennas. Thealways-available (and non-configurable) common pilots may be used forvarious tasks such as pilot measurements, demodulation for datatransmission sent at low to medium rates, etc. Supplemental (andconfigurable) common pilots may be sent on one or more additionalantennas when there are scheduled downlink transmissions that make useof the additional antenna(s).

In general, the common pilots may be sent at a lower density and/or alower transmit power level in order to mitigate pilot pollution. Reducedcommon pilots are common pilots sent at lower density and/or lowertransmit power. Lower density may be achieved by sending the commonpilots less frequently, on fewer subcarriers, and/or from fewerantennas. Pilot reduction may be applicable to any common pilot such asthe cell-specific reference signal, the synchronization signals, etc. Inone design, a cell may reduce the density and/or transmit power of itscommon pilots without informing the terminals. In another design, thecell may broadcast the format of the reduced common pilots on a controlchannel to the terminals.

In one design, adjacent cells may send reduced common pilots using timedivision multiplexing (TDM), frequency division multiplexing (FDM), orsome other multiplexing scheme. The common pilots from each cell maythus avoid the common pilots and data from neighbor cells. In one designof TDM, the transmission timeline may be partitioned into intervals, andeach cell may send its common pilots and data in intervals reserved forthat cell. In one design of FDM, the system bandwidth may be partitionedinto non-overlapping subcarrier sets, with each set including contiguousor non-contiguous subcarriers not included in other sets. Each cell maysend its common pilots and data on a subcarrier set reserved for thatcell. The multiplexing may be dependent on the manner in which thedensity of the common pilots is reduced. For example, TDM may be used ifthe common pilots are sent less frequently, and FDM may be used if thecommon pilots are sent on fewer subcarriers. The multiplexing of commonpilots from adjacent cells may further mitigate pilot pollution.

The common pilots may be used for various purposes and tasks, asdescribed above. Reducing the density and/or transmit power of thecommon pilots may adversely impact performance. Performance degradationmay be mitigated in various manners, as described below.

The common pilots may be used as a reference signal for serving cellselection. Reducing the density and/or transmit power of the commonpilots may impact measurements of signal strength and/or channelquality, which may result in selection of a cell transmitting the commonpilots at higher density and/or higher transmit power. In one design, acommon pilot used for serving cell selection (e.g., the primary and/orsecondary synchronization signal) may be sent at a constant transmitpower level. The common pilots not used for serving cell selection(e.g., the cell-specific reference signal) may be sent at a lowertransmit power level. In another design, a low overhead pilot may besent at a constant transmit power level and used for serving cellselection. The low overhead pilot may be sent less frequently (e.g.,once every radio frame or every N-th radio frames) than other commonpilots and/or on fewer subcarriers in order to reduce overhead.

The common pilots may be used for channel estimation, and a channelestimate may be used for demodulation and/or decoding of data and/orcontrol transmissions. Reducing the density and/or transmit power of thecommon pilots may degrade the channel estimate, which may in turndegrade decoding performance. In one design, a modulation and codingscheme (MCS) may be selected by taking into account degradation inchannel estimation due to reduced common pilots. The selected MCS mayhave a lower code rate and/or a lower order modulation scheme due to adegraded channel estimate. In another design, data and controlinformation for a terminal may be sent as close to the common pilots aspossible in order to reduce performance degradation.

Correct decoding of control information may be important, e.g., in orderto properly receive and process data. The common pilots used fordemodulating and decoding control information may be maintained at asufficient level in order to ensure good decoding performance forcontrol information. In one design, control information and data may besent in different time regions with TDM. In this design, the commonpilots in a control region may be maintained or slightly reduced whereasthe common pilots in a data region may be shut off or more significantlyreduced.

The common pilots may be used for CQI measurement and reporting. Since aCQI report may be sent infrequently, the density and/or transmit powerof the common pilots may be reduced while minimally impacting CQIreporting. In one design, a certain minimum level of common pilots maybe sent to support CQI measurement and reporting pilot.

FIG. 4 shows a design of a process 400 for sending common pilots in awireless network. Process 400 may be performed by a cell (as describedbelow) or some other entity. The cell may send a common pilot at a firstdensity and a first transmit power level during a first time period toterminals within detection range of the cell (block 412). The cell maysend the common pilot at a second density and a second transmit powerlevel during a second time period (block 414). The second density may belower than the first density and/or the second transmit power level maybe lower than the first transmit power level. The common pilot maycomprise a cell-specific reference signal used for channel estimationand other purposes by terminals, a synchronization signal used for cellacquisition and selection by the terminals, and/or other signals usedfor other purposes.

The cell may determine whether to reduce the density and/or transmitpower of the common pilot based on network loading, offered loading at acell, measured pilot pollution by terminals, SINRs of terminals, pilotrequirements of terminals, etc. (block 416). For example, the networkloading may be light during the second time period and not light duringthe first time period. As another example, the SINRs of the terminalsmay be high during the second time period and lower during the firsttime period. In another design, the first time period may be for controlinformation and the second period may be for data. The cell may sendcontrol information during the first time period and may send dataduring the second time period. In any case, the cell may sendinformation indicating a format of the common pilot.

In one design, the cell may send the common pilot at a lower density butthe same transmit power level during the second time period as comparedto the first time period. To achieve lower density, the cell may sendthe common pilot less frequently, or on fewer subcarriers, or from fewerantennas, or a combination thereof. In another design, the cell may sendthe common pilot at the same density but a lower transmit power levelduring the second time period as compared to the first time period.

In one design, the cell may select a first MCS for a terminal during thefirst time period and may select a second MCS for the terminal duringthe second time period. The common pilot may be used for channelestimation by the terminal. The second MCS may be lower than the firstMCS to account for degradation in channel estimation during the secondtime period. The cell may send data near the common pilot during thesecond time period in order to improve decoding performance for thedata.

In one design, the cell may send a second common pilot at a constanttransmit power level and lower density (e.g., less frequently) than thecommon pilot mentioned above. The second common pilot may be used by theterminals for serving cell selection and/or other purposes.

In one design, adjacent cells may send their common pilots in differentintervals during the second time period with TDM. In another design, theadjacent cells may send their common pilots on different sets ofsubcarriers during the second time period with FDM. The adjacent cellsmay also send their common pilots based on other multiplexing schemes.

FIG. 5 shows a design of an apparatus 500 for sending common pilots in awireless network. Apparatus 500 includes a module 512 to send a commonpilot at a first density and a first transmit power level during a firsttime period to terminals within detection range of a cell, a module 514to send the common pilot at a second density and a second transmit powerlevel during a second time period, with the second density being lowerthan the first density and/or the second transmit power level beinglower than the first transmit power level, and a module 516 to determinewhether to reduce the density and/or transmit power of the common pilotbased on network loading, SINRs of terminals, etc.

FIG. 6 shows a design of a process 600 for receiving common pilots in awireless network. Process 600 may be performed by a terminal (asdescribed below) or some other entity. The terminal may receive a commonpilot sent by a cell at a first density and a first transmit power levelduring a first time period (block 612). The terminal may receive thecommon pilot sent by the cell at a second density and a second transmitpower level during a second time period (block 614). The second densitymay be lower than the first density and/or the second transmit powerlevel may be lower than the first transmit power level. The terminal mayreceive information indicating the format of the common pilot from thecell and may receive the common pilot in accordance with the format.

In one design, the terminal may perform channel estimation based on thecommon pilot. The terminal may receive data sent by the cell inaccordance with a first MCS during the first time period and may receivedata sent by the cell in accordance with a second MCS during the secondtime period. The second MCS may be lower than the first MCS to accountfor degradation in channel estimation during the second time period.

In one design, the terminal may receive a second common pilot sent bythe cell at a constant transmit power level and lower density (e.g.,less frequently) than the common pilot. The terminal may use the firstand/or second common pilot to select a serving cell.

FIG. 7 shows a design of an apparatus 700 for receiving common pilots ina wireless network. Apparatus 700 includes a module 712 to receive acommon pilot sent by a cell at a first density and a first transmitpower level during a first time period, and a module 714 to receive thecommon pilot sent by the cell at a second density and a second transmitpower level during a second time period, with the second density beinglower than the first density and/or the second transmit power levelbeing lower than the first transmit power level.

In another aspect, pilot pollution may be mitigated by performing pilotcancellation at a terminal A common pilot from a neighbor cell mayinterfere with a common pilot and/or data from a serving cell. Theterminal may estimate interference due to the common pilot from theneighbor cell and may cancel the estimated interference from a receivedsignal at the terminal. In general, the terminal may estimate and cancelinterference due to any set of common pilots sent by any number ofneighbor cells. The terminal may process an interference-canceled signalto recover data and/or other information from the serving cell. Pilotcancellation may be performed for common pilots sent in the normalmanner as well as for reduced common pilots sent at lower density and/orlower transmit power.

Pilot cancellation may be performed in various manners. In one design, aterminal may perform pilot cancellation for M strongest neighbor cells,where M may be a value of one or greater. The terminal may measure thereceived signal strength of all neighbor cells, rank the neighbor cellsbased on their received signal strength, and select the M strongestneighbor cells to perform pilot cancellation.

In another design, the terminal may perform pilot cancellation for eachneighbor cell having sufficiently strong received signal strength at theterminal and considered as a dominant interferer to the terminal. Aneighbor cell may be considered as a dominant interferer if its receivedsignal strength exceeds a threshold, which may be a fixed value or aconfigurable value. A configurable threshold may be determined based ontotal interference and noise at the terminal. The terminal may performpilot cancellation for a variable number of neighbor cells in thisdesign.

In yet another design, the terminal may perform pilot cancellation basedon SINR and/or network loading. For example, the terminal may performpilot cancellation for high SINR scenarios (e.g., if a required SINR forthe terminal exceeds a threshold) and may skip pilot cancellation forlow SINR scenarios. The terminal may also perform pilot cancellationonly if the network loading is light and interference at the terminal isdominant by the common pilots from the neighbor cells.

In yet another design, the terminal may perform pilot cancellation asindicated by the serving cell. For example, the serving cell may informthe terminal of which neighbor cells to search for, which common pilotsto detect, the format of each common pilot, etc. The terminal may thenperform pilot cancellation for the neighbor cells and/or the commonpilots indicated by the serving cell.

The terminal may perform pilot cancellation based on any one or anycombination of the designs described above. These designs may reducedevice cost and/or power consumption associated with pilot cancellation.

The terminal may perform pilot cancellation for a common pilot from aneighbor cell as follows. The terminal may locally generate the commonpilot from the neighbor cell and may correlate a received signal withthe locally generated common pilot. This processing may be referred toas common pilot demodulation. The terminal may then process the resultsof the common pilot demodulation to obtain a channel estimate for theneighbor cell. The terminal may apply the channel estimate to thelocally generated common pilot to estimate interference due to thecommon pilot. The terminal may then subtract the estimated interferencefrom the received signal to obtain an interference-canceled signal. Theterminal may repeat the process for each common pilot of each neighborcell to be canceled. Pilot cancellation may also be performed in othermanners.

The terminal may perform common pilot demodulation for a neighbor cellfor various tasks such as control channel demodulation, signal strengthmeasurement, etc. In this case, pilot cancellation may be an additionalstep that makes use of the results of the common pilot demodulation.Alternatively, the terminal may perform common pilot demodulation justfor pilot cancellation.

In one design, adjacent cells may send their common pilots on the sameresources, and the common pilots from these cells may overlap oneanother in time and frequency. In this design, the terminal may performjoint channel estimation for the serving cell as well as one or moreneighbor cells, e.g., using minimum mean square error (MMSE) techniques.The terminal may obtain channel estimates of higher quality using jointestimation. The terminal may observe lower pilot SINR compared to dataSINR, especially when the network loading is light and in the absence ofpilot cancellation. Performance may then be dominated by poor channelestimation for terminals located at cell edge and observing high pilotinterference from one or more strong interfering cells. The edgeterminals may perform pilot cancellation in order to improveperformance.

In another design, adjacent cells may send their common pilots ondifferent resources, which may be randomized across different cells. Inthis design, the common pilots and data from each cell may observesimilar average SINR. Pilot pollution may degrade both pilot and dataSINRs and may define an SINR ceiling when network loading is light,e.g., when the network loading is comparable to or lower than theoverhead of the common pilots. Pilot cancellation may be used to improveboth pilot and data SINRs.

FIG. 8 shows a design of a process 800 for performing pilotcancellation. Process 800 may be performed by a terminal (as describedbelow) or some other entity. The terminal may determine whether toperform pilot cancellation for a neighbor cell (block 812). The terminalmay cancel a common pilot from the neighbor cell if a determination ismade to perform pilot cancellation (block 814). The common pilot may besent on resource elements scattered across time and frequency, e.g., asshown in FIGS. 3A and 3B. The terminal may determine received signalquality for a serving cell after canceling the common pilot (block 816).The terminal may determine CQI information based on the received signalquality (block 818). The CQI information may comprise one or more SINRestimates, one or more modulation and coding schemes, etc. The terminalmay send the CQI information to the serving cell, which may use the CQIinformation for data transmission to the terminal (block 820).

In one design of block 812, the terminal may decide to perform pilotcancellation if the neighbor cell is one of M strongest neighbor cellsor if received signal strength for the neighbor cell exceeds athreshold. In another design, the terminal may determine whether toperform pilot cancellation based on network loading, a required SINR ofthe terminal, and/or other criteria. In yet another design, the terminalmay determine whether to perform pilot cancellation based on informationreceived from the serving cell. The information may identify at leastone neighbor cell to perform pilot cancellation and/or at least onecommon pilot to perform pilot cancellation.

In one design of block 814, the terminal may obtain a channel estimatefor the neighbor cell based on a received signal at the terminal. Theterminal may then estimate interference due to the common pilot from theneighbor cell based on a locally generated common pilot for the neighborcell and the channel estimate. The terminal may then subtract theestimated interference from the received signal to obtain aninterference-canceled signal. The terminal may perform pilotcancellation for other common pilots from the neighbor cell and/or fromother neighbor cells.

FIG. 9 shows a design of an apparatus 900 for performing pilotcancellation. Apparatus 900 includes a module 912 to determine whetherto perform pilot cancellation for a neighbor cell, a module 914 tocancel a common pilot from the neighbor cell at a terminal if adetermination is made to perform pilot cancellation, a module 916 todetermine received signal quality for a serving cell at the terminalafter canceling the common pilot, a module 918 to determine CQIinformation based on the received signal quality, and a module 920 tosend the CQI information to the serving cell.

The modules in FIGS. 5, 7 and 9 may comprise processors, electronicsdevices, hardware devices, electronics components, logical circuits,memories, etc., or any combination thereof.

In another design, the terminal may perform pilot cancellation but maycompute SINR without taking into account pilot cancellation, i.e., withun-canceled interference. The computed SINR may be worse than the actualSINR with pilot cancellation. The terminal may determine CQI informationbased on the computed SINR without pilot cancellation and may send theCQI information to the serving cell. The serving cell may have knowledgethat the terminal performs pilot cancellation and may send data to theterminal by taking into account the difference between the actual andreported SINRs. For example, the serving cell may send a number of HARQtransmissions for a packet to the terminal and may target a later H-ARQtermination. In particular, the serving cell may select a modulation andcoding scheme based on the reported SINR such that the terminal canrecover the packet with a target number of HARQ transmissions. Since theactual SINR at the terminal may be better than the reported SINR, theterminal may be able to recover the packet with fewer HARQ transmissionsand may terminate earlier than the HARQ termination target, as expectedby the serving cell.

The pilot reduction and pilot cancellation techniques described hereinmay improve performance for wireless networks operating at partialloading most of the time. This kind of network deployment may allownetwork operators to ensure sufficiently high capacity at peak load(which may occur infrequent) while providing superior user experiencedue to higher data rates and lower latencies most of the time whennetwork load stays relatively light. The techniques may also improveperformance for certain operating scenarios, e.g., high SINR scenarios.

FIG. 10 shows a block diagram of a design of base station 110 andterminal 120, which may be one of the base stations and one of theterminals in FIG. 1. In this design, base station 110 is equipped with Tantennas 1034 a through 1034 t, and terminal 120 is equipped with Rantennas 1052 a through 1052 r, where in general T≧1 and R≧1.

At base station 110, a transmit processor 1020 may receive data for oneor more terminals from a data source 1012, process (e.g., encode,interleave, and modulate) the data for each terminal based on one ormore modulation and coding schemes selected for that terminal, andprovide data symbols for all terminals. Transmit processor 1020 may alsoprocess control information and provide control symbols. Transmitprocessor 1020 may also generate the common pilots for each cell andprovide pilot symbols for all cells at the base station. A transmit (TX)multiple-input multiple-output (MIMO) processor 1030 may perform spatialprocessing (e.g., precoding) on the data symbols, the control symbols,and/or the pilot symbols, if applicable, and may provide T output symbolstreams to T modulators (MODs) 1032 a through 1032 t. Each modulator1032 may process a respective output symbol stream (e.g., for OFDM) toobtain an output sample stream. Each modulator 1032 may further process(e.g., convert to analog, amplify, filter, and upconvert) the outputsample stream to obtain a downlink signal. T downlink signals frommodulators 1032 a through 1032 t may be transmitted via T antennas 1034a through 1034 t, respectively.

At terminal 120, antennas 1052 a through 1052 r may receive the downlinksignals from base station 110 and provide received signals todemodulators (DEMODs) 1054 a through 1054 r, respectively. Eachdemodulator 1054 may condition (e.g., filter, amplify, downconvert, anddigitize) a respective received signal to obtain input samples and mayfurther process the input samples (e.g., for OFDM) to obtain receivedsymbols. A MIMO detector 1056 may obtain received symbols from all Rdemodulators 1054 a through 1054 r, perform MIMO detection on thereceived symbols if applicable, and provide detected symbols. A receiveprocessor 1058 may process (e.g., demodulate, deinterleave, and decode)the detected symbols, provide decoded data to a data sink 1060, andprovide decoded control information to a controller/processor 1080. Acommon pilot processor 1070 may process the common pilots from theserving and neighbor cells, e.g., for channel estimation, channelquality measurement, signal strength measurement, time/frequencytracking, etc.

On the uplink, at terminal 120, data from a data source 1062 and controlinformation from controller/processor 1080 may be processed by atransmit processor 1064, further processed by a TX MIMO processor 1066if applicable, conditioned by modulators 1054 a through 1054 r, andtransmitted to base station 110. At base station 110, the uplink signalsfrom terminal 120 may be received by antennas 1034, conditioned bydemodulators 1032, processed by a MIMO detector 1036 if applicable, andfurther processed by a receive processor 1038 to obtain the data andcontrol information sent by terminal 120.

Controllers/processors 1040 and 1080 may direct the operation at basestation 110 and terminal 120, respectively. Processor 1040 and/or otherprocessors and modules at base station 110 may perform or direct process400 in FIG. 4 and/or other processes for the techniques describedherein. Processor 1080 and/or other processors and modules at terminal120 may perform or direct process 600 in FIG. 6, process 800 in FIG. 8,and/or other processes for the techniques described herein. Memories1042 and 1082 may store data and program codes for base station 110 andterminal 120, respectively. A scheduler 1044 may schedule terminals fordownlink and/or uplink transmission and may provide assignments ofresources for the scheduled terminals.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the disclosure herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thedisclosure herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method for wireless communication, comprising:determining whether to perform pilot cancellation at a terminal for aneighbor cell based on network loading, or a requiredsignal-to-noise-and-interference ratio (SINR) of the terminal, or both;and when a determination is made to perform pilot cancellation,canceling a common pilot from the neighbor cell at the terminal,determining received signal quality of a serving cell at the terminalafter canceling the common pilot, determining channel quality indicator(CQI) information based on the received signal quality, and sending theCQI information to the serving cell.
 2. The method of claim 1, whereinthe determining whether to perform pilot cancellation comprises decidingto perform pilot cancellation if the neighbor cell is one of M strongestneighbor cells or if received signal strength for the neighbor cellexceeds a threshold, where M is one or greater.
 3. The method of claim1, wherein the determining whether to perform pilot cancellationcomprises determining whether to perform pilot cancellation based oninformation received from a serving cell, the information identifying atleast one neighbor cell to perform pilot cancellation, or at least onecommon pilot to perform pilot cancellation, or both.
 4. The method ofclaim 1, wherein the canceling the common pilot comprises obtaining achannel estimate for the neighbor cell based on a received signal at theterminal, estimating interference due to the common pilot from theneighbor cell based on a locally generated common pilot for the neighborcell and the channel estimate, and subtracting the estimatedinterference from the received signal to obtain an interference-canceledsignal.
 5. The method of claim 1, including: receiving at the terminal acommon pilot at a first density and a first transmit power level duringa first time period; receiving at the terminal the common pilot at asecond density and a second transmit power level during a second timeperiod, the second density being lower than the first density, or thesecond transmit power level being lower than the first transmit powerlevel, or both; using a first modulation and coding scheme (MCS) at theterminal during the first time period; and using a second MCS at theterminal during the second time period, wherein the common pilot is usedfor channel estimation by the terminal, and wherein the second MCS islower than the first MCS to account for degradation in channelestimation during the second time period; wherein the first and secondtime periods are determined based on SINRs of terminals, the SINRs beinghigh during the second time period and not high during the first timeperiod.
 6. The method of claim 1, wherein the CQI information includesan SINR estimate.
 7. The method of claim 1, wherein the CQI informationincludes a modulation and coding scheme.
 8. An apparatus for wirelesscommunication, comprising: a memory coupled to at least one processor;and the at least one processor configured to determine whether toperform pilot cancellation for a neighbor cell based on network loading,or a required signal-to-noise-and-interference ratio (SINR) of theapparatus, or both, and, when a determination is made to perform pilotcancellation, to cancel a common pilot from the neighbor cell, todetermine received signal quality of a serving cell after canceling thecommon pilot, to determine channel quality indicator (CQI) informationbased on the received signal quality, and to send the CQI information tothe serving cell.
 9. The apparatus of claim 8 wherein the at least oneprocessor is configured to decide to perform pilot cancellation if theneighbor cell is one of M strongest neighbor cells or if received signalstrength for the neighbor cell exceeds a threshold, where M is one orgreater.
 10. The apparatus of claim 8, wherein the at least oneprocessor is configured to obtain a channel estimate for the neighborcell based on a received signal at the apparatus, to estimateinterference due to the common pilot from the neighbor cell based on alocally generated common pilot for the neighbor cell and the channelestimate, and to subtract the estimated interference from the receivedsignal to obtain an interference-canceled signal.
 11. The apparatus ofclaim 8, wherein the CQI information includes an SINR estimate.
 12. Theapparatus of claim 8, wherein the CQI information includes a modulationand coding scheme.
 13. The apparatus of claim 8, wherein the at leastone processor is configured to determine whether to perform pilotcancellation based on information received from a serving cell, theinformation identifying at least one neighbor cell to perform pilotcancellation, or at least one common pilot to perform pilotcancellation, or both.
 14. An apparatus for wireless communication,comprising: means for determining whether to perform pilot cancellationfor a neighbor cell based on network loading, or a requiredsignal-to-noise-and-interference ratio (SINR) of the apparatus, or both;and means operable, when a determination is made to perform pilotcancellation, for canceling a common pilot from the neighbor cell,determining received signal quality of a serving cell after cancelingthe common pilot, determining channel quality indicator (CQI)information based on the received signal quality, and means for sendingthe CQI information to the serving cell.
 15. The apparatus of claim 14,wherein the determining whether to perform pilot cancellation comprisesdeciding to perform pilot cancellation if the neighbor cell is one of Mstrongest neighbor cells or if received signal strength for the neighborcell exceeds a threshold, where M is one or greater.
 16. The apparatusof claim 14, wherein the canceling the common pilot includes obtaining achannel estimate for the neighbor cell based on a received signal at theapparatus, estimating interference due to the common pilot from theneighbor cell based on a locally generated common pilot for the neighborcell and the channel estimate, and subtracting the estimatedinterference from the received signal to obtain an interference-canceledsignal.
 17. The apparatus of claim 14, wherein the determining whetherto perform pilot cancellation comprises determining whether to performpilot cancellation based on information received from a serving cell,the information identifying at least one neighbor cell to perform pilotcancellation, or at least one common pilot to perform pilotcancellation, or both.
 18. The apparatus of claim 14, wherein the CQIinformation includes an SINR estimate.
 19. The apparatus of claim 14,wherein the CQI information includes a modulation and coding scheme. 20.The apparatus of claim 14, including: means for receiving a common pilotat a first density and a first transmit power level during a first timeperiod; means for receiving the common pilot at a second density and asecond transmit power level during a second time period, the seconddensity being lower than the first density, or the second transmit powerlevel being lower than the first transmit power level, or both; meansfor using a first modulation and coding scheme (MCS) during the firsttime period; and means for using a second MCS during the second timeperiod, wherein the common pilot is used for channel estimation by theapparatus, and wherein the second MCS is lower than the first MCS toaccount for degradation in channel estimation during the second timeperiod; wherein the first and second time periods are determined basedon SINRs of terminals, the SINRs being high during the second timeperiod and not high during the first time period.